1. Field of the Invention
The present invention relates in general to video signal processing systems in which an input video signal is compressed for transmission and the compressed video signal is restored to be displayed on a screen, and more particularly to a motion compensating apparatus which is capable of restoring a video signal even when a channel change or a channel error is generated.
2. Description of the Prior Art
Referring to FIG. 1, there is shown a block diagram of a conventional motion compensating apparatus. As shown in this drawing, the conventional motion compensating apparatus comprises an adder 1 for adding a multiplied signal .mu.g'(t) to a video signal g(t) of a present frame to output a video signal y(t) of a displaced frame difference (DFD) frame which is a difference between the video signal g(t) of the present frame and a motion compensated video signal g'(t), a discrete cosine transform (DCT) unit 2 for transforming a time domain of the video signal y(t) of the DFD frame from the adder 1 into a frequency domain, a quantizer 3 for quantizing an output signal from the DCT unit 2, and a variable length coder (VLC)/buffer 4 for compressing video signal blocks from the quantizer 3 by different lengths according to a probability that the video signal blocks occur and outputting a step size SS, a threshold TH, a scaling factor SF and a DF scaling factor DSF according to statistical properties of an input video signal.
The conventional motion compensating apparatus also comprises a reverse quantizer 5 for quantizing reversely an output signal from the quantizer 3, a reverse DCT unit 6 for transforming a frequency domain of an output signal from the reverse quantizer 5 into a time domain, an adder 7 for adding the multiplied signal .mu.g'(t) to an output signal from the reverse DCT unit 6 to output a video signal g'(t-1) of a previous frame, a motion compensator 8 for calculating a motion vector MV from the video signal g'(t-1) of the previous frame from the adder 7 and outputting the motion compensated video signal g'(t) based on the calculated motion vector MV, and a multiplier 9 for multiplying the motion compensated video signal g'(t) from the motion compensator 8 by the DF scaling factor DSF (.mu.) to output the multiplied signal .mu. g'(t).
Here, the reference numerals .epsilon.q and .epsilon.m, not described, designate an error generated in the quantizer 3 and an estimated motion error, respectively.
The operation of the conventional motion compensating apparatus with the above-mentioned construction will hereinafter be described.
To be brief, the video signal of the previous frame is partitioned into blocks, and it is then estimated that each block is moved to any one of blocks of the video signal of the present frame. The estimated motion is expressed as a vector. In result, the original video signal is restored by using the motion vector MV and the video signal y(t) of the DFD frame which is the difference between the video signal g(t) of the present frame and the motion compensated video signal g'(t).
In detail, upon inputting the video signal g(t) of the present frame, the motion compensator 8 calculates a difference between the inputted video signal of the present frame g(t) and the video signal g'(t-1) of the previous frame stored therein and compensates for the video signal g'(t-1) of the previous frame by the calculated difference. As a result of the compensation, the motion compensator 8 outputs the video signal g'(t) analogous to the video signal g(t) of the present frame to the multiplier 9.
The adder 1 obtains the video signal y(t) of the DFD frame which is the difference between the video signal g(t) of the present frame and the video signal g'(t) analogous thereto. This video signal y(t) of the DFD frame from the adder 1 is transmitted through the DCT unit 2, the quantizer 3 and the variable length coder/buffer 4. On the basis of the motion vector and the statistical properties of the video signal of the DFD frame, the variable length coder/buffer 4 outputs the step size SS, the threshold TH and the scaling factor SF to control the quantizer 3. Also, the variable length coder/buffer 4 outputs the DF scaling factor DSF to reduce an error component being generated in performing the motion compensation to the minimum.
On the other hand, the video signal of the DFD frame quantized by the quantizer 3 is reversely quantized by the reverse quantizer 5 and the frequency domain thereof is transformed into the time domain by the reverse DCT unit 6. The video signal of the DFD frame from the reverse DCT unit 6 is applied to the motion compensator 8. On the basis of the video signal of the DFD frame, the motion compensator 8 compensates again for the motion compensated video signal g'(t) analogous to the video signal of the present frame and then stores the motion re-compensated video signal as a video signal of a reference frame therein.
Thereafter, upon inputting the video signal g(t) of the present frame, the motion compensator 8 calculates a difference between the inputted video signal of the present frame g(t) and the video signal g'(t-1) of the previous frame stored therein and compensates for the video signal g'(t-1) of the previous frame by the calculated difference. As a result of the compensation, the motion compensator 8 outputs the video signal g'(t) analogous to the video signal g(t) of the present frame to the multiplier 9.
The multiplier 9 multiplies the motion compensated video signal g'(t) from the motion compensator 8 by the DF scaling factor DSF (.mu.) and outputs the multiplied signal .mu.g'(t) to the adder 1, which obtains the video signal y(t) of the DFD frame which is the difference between the video Signal g(t) of the present frame and the video signal .mu.g'(t) analogous thereto. This video signal y(t) of the DFD frame from the adder 1 is frequency domain-transformed and quantized by the DCT unit 2 and the quantizer 3, respectively. The quantized video signal y(t) is compressed for transmission by the variable length coder/buffer 4.
On the other hand, the video signal of the DFD frame quantized by the quantizer 3 is reversely quantized by the reverse quantizer 5 and the frequency domain thereof is transformed into the time domain by the reverse DCT unit 6. The video signal of the DFD frame from the reverse DCT unit 6 is applied to the motion compensator 8, which compensates again for the motion compensated video signal g'(t) analogous to the video signal of the present frame on the basis of the video signal of the DFD frame and then stores the motion recompensated video signal as the video signal of the reference frame therein.
In the motion compensator 8, the video signal of the previous frame is partitioned into blocks, and it is then estimated that each block is moved to any one of blocks of the video signal g(t) of the present frame. The motion of the blocks of the video signal of the previous frame is compensated for on the basis of the estimated motion vector MV. At this time, the motion vector MV is compressed for transmission, and the motion compensated video signal g'(t) analogous to the video signal of the present frame is stored in the motion compensator 8 and also applied to the multiplier 9, which multiplies the applied motion compensated video signal g'(t) by the DF scaling factor DSF (.mu.) and outputs the multiplied signal .mu.g'(t) to the adder 1.
Obtained in the adder I is the video signal y(t) of the DFD frame which is the difference between the video signal .mu.g'(t) of the present frame and the video signal g'(t) the motion of which was compensated for on the basis of the video signal of the present frame. This video signal y(t) of the DFD frame from the adder 1 is compressed for transmission by the DCT unit 2, the quantizer 3 and the variable length codes/buffer 4. Namely, there is transmitted the video signal of the DFD frame which is the difference between the video signal .mu.g'(t) of the present frame and the video signal g'(t) which is analogous to the video signal of the present frame and the motion thereof was compensated for by the motion compensator 8. Therefore, the original video signal is restored on the basis of the video signal of the reference frame and the motion vector MV from the motion compensator 8 and the video signal y(t) of the DFD frame from the adder 1.
In the conventional motion compensating apparatus, however, in the case where the video signal of the reference frame stored in the motion compensator is subjected to a loss due to a channel error, the original video signal can be restored to a certain degree on the basis of the multiplied signal, but a good long time is required in restoring the original video signal because of a linearity of the scaling factor of the multiplied signal, particularly when a great number of errors are generated in the motion compensation. Also, the video signal of the reference frame is forced to be refreshed at predetermined time intervals. This results in an increase in the time being required in compensating for the motion.